Method of preparing magnesium hydroxide



United States Patent m 3,451,774 METHOD OF PREPARING MAGNESIUM HYDROXIDE Ivan M. Thompson and John Maskal, Ludington, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed June 1, 1966, Ser. No. 554,480 Int. Cl. C01d /14 US. Cl. 23--201 3 Claims ABSTRACT OF THE DISCLOSURE An improved method of preparing magnesium hydroxide formed from dolomitic stone and magnesium chloride containing brine which comprises continued con- This invention relates to a new and novel method of preparing magnesium hydroxide from dolomitic stone and brines, and more particularly relates to an improved method of preparing dewaterable, pumpable and repulpable magnesium hydroxide slurries which upon drying have a higher than ordinary surface area and which,

upon calcining, provide a magnesium oxide product having a high apparent density.

In one conventional method of preparing periclase grade magnesium hydroxide slurries from dolomite and from magnesium chloride-containing brines, e.g., as taught in US. Patent 3,080,215, dolomite (a substantially equimolar mixture of calcium carbonate and magnesium carbonate) is first crushed to small calcinable pieces then calcined at, e.g., about 1300 C. to the corresponding oxides by driving ofi? carbon dioxide. This calcined dolomite is referred to hereinafter as dolime. Socalcined, the calcium oxide portion of the dolime is slaked, i.e., reacted with, e.g., a 9 percent to 15 percent aqueous calcium chloride-containing solution to convert said cal cium oxide to the hydroxide. The slaked dolime is used as a precipitant later in the process. Such slaking may be carried out, for example, at a temperature of from about 70 C. to about 100 C. for a time approximately from about 1 to about 30 minutes. Though some slaking of the magnesium oxide in the dolime takes place at the time of the CaO slaking, substantially most of the MgO remains as such. This is subsequently slaked or hydrated during the course of traversing the various steps of the process (one of which is a specific MgO slaking or bydrating step). The slaked dolime is next reacted with the aforesaid magnesium chloride-containing brine to precipitate said magnesium chloride as a slurry of magnesium hydroxide in an aqueous calcium chloride mother liquor. The calcium chloride in the mother liquor derives primarily from the initial concentration thereof in the brine and also as a reaction by-productof the calcium hydroxide precipitant and magnesium chloride. So precipitated, the solids comprising magnesium hydroxide (and unconverted magnesium oxide) are separated from the mother liquor of the precipitator slurry as by filtration. The mother liquor is normally discarded after said separation, or used to recover the CaCl therefrom, since it is no longer useful in the process. Washing and filtering of the magnesium hydroxide and the remaining magnesium oxide solids with water follows in order to re- 3,451,774 Patented June 24, 1969 move the chloride and other soluble impurities therefrom. These solids are then held in a holding tank wherein steam is sparged therethrough to hydrate the MgO to Mg(OH) Upon again refiltering and reslurrying, the solids are creamed or repulped in agitated tanks to homogenize or break up the filter cake and agglomerates therein to a uniform slurry consistency. The total magnesium hydroxide values (i.e., those obtained by precipitation from the brine and those obtained from the conventional subsequent slaking of the MgO) are ready for production into periclase or to some other magnesium hydroxide or oxide product.

The magnesium hydroxide slurries obtained from these prior art methods cannot, even with repetitive filtering and repulping operations, normally be dewatered or concentrated to a pumpable percent solids exceeding about 59 to 60 percent. Moreover, the periclase produced therefrom upon calcining exhibits a significant density obtainable in accordance with the present invention.

These latter factors are important since there has been a continuing demand in recent years by periclase refractory manufacturers, for a supply of dewaterable and repulpabe magnesium hydroxide slurries of such a solids content and character as will enable more economical shipment, and a magnesium hydroxide from which a higher resulting periclase density can be obtained than has heretofore been avail-able.

A principal object of the present invention, therefore, is to provide a new and improved method of preparing a magnesium hydroxide slurry, having a higher solids content than realized with conventional processes practiced he-retofore and from which a higher density periclase product can be derived than heretofore available from a dolomitic-brine process.

The appended drawing shows surface area values and ignition loss values of precipitator solids treated in accordance with one embodiment of the present invention in connection with Example IV hereinafter.

In general, in a process as described. above, the novel improvement of the present invention comprises: treating precipitator solids, namely magnesium oxide values, in the presence of an aqueous calcium chloride mother liquor of the precipitator slurry, but outside the precipitator tank, for a duration of time and at a temperature sufficient to convert substantially all the magnesium oxide values to the corresponding hydroxide. It is essential that this treatment of the MgO according to the present novel process be accomplished in the presence of said calcium chloride-containing mother liquor, but at a point in the process other than during the precipitation of the magnesium values in the aforesaid brine, that is other than in the precipitator tank.

Treatment of the MgO in accordance with the present invention may be carried out, for example, in a suitable reactor subsequent to the precipitation step (i.e., subsequent to precipitation of the magnesium values in the brine), but prior to separation of any solids from the mother liquor. An operable treatment temperature is Within the range of from about 25 C. to about 100 C., and preferably from about 45 C. to about C. The range of time required to accomplish this treatment varies from about 4 to 8 hours, up to from about 2 to about 5 days, depending on an inverse relationship basis to the temperature being employed. For example, at C. the MgO treatment may require from 4 to 8 hours, while at 25 C. it may require from 3 to 5 days. A particularly preferred temperature and time of operation, at 60-65 C., is from about 12 to 25 hours. Further, it is necessary that the MgO treatment be accomplished in a calcium chloride aqueous medium containing at least about 1 percent calcium chloride and up to about 30 percent, but preferably from about a to 30 percent calcium chloride concentration.

This calcium chloride treatment aids in obtaining a higher surface area of the magnesium hydroxide and upon calcination produces a higher density MgO. Moreover, a final pumpable slurry is provided having a higher solids percent than realized heretofore.

It is manifest that various combinations of calcium chloride concentration, temperature, and treatment time of the MgO, may be predetermined within the disclosed range as best suited for and employed in actually carrying out the improved process of the present invention with a given slurry in order to prepare the improved magnesium hydroxide described hereinbefore.

The present improvement in magnesium hydroxide processing can be employed with any slurry resulting from any of a variety of dolomite or brine precipitations. The present invention particularly is suitable for use with slurries produced in accordance with the teaching of US. 3,080,215, described hereinbefore to recover a dewaterable and high surface area magnesium hydroxide product therefrom. The present invention, however, is not to be construed as being limited to reactant sources from said processes.

The following examples serve to further illustrate the operation and utility of the present invention, but are not to be construed in any way as limiting the invention thereto.

EXAMPLE I A large initial sample of conventionally prepared precipitator slurry was obtained, containing magnesium hydroxide, unslaked MgO, about 20 percent by weight calcium chloride, and water. The precipitator slurry had been prepared by first crushing Cedarville dolomite stone to calcinable pieces and calcining same at about 1350 C. to dolime, then slaking the dolime at about 81 C. in a 10 percent aqueous calcium chloride slaking liquid. So slaked, the dolime was reacted with a brine assaying by weight as follows:

Percent CaCl 17.4 MgCl 9.8 NaCl 2.9 Water 69.9

Trace amounts of other alkali metal and alkaline earth halogenated salts included.

The reaction was carried out at a temperature of about Part (a) of each said subportion of precipitator slurry was filtered and the solids washed with water to reduce the chloride content to the level indicated in Table I below. The solids were then reslurried and repulped to a percent solids level in water then one sample heated to a temperature of 100 C. for 2 /2 days, one at 80 C. for 3 /2 days, one at C. for 6 /2 days, one at 45 C. for 7 /2 days, and one at 25 C. for 15 /2 days, for the purpose of water slaking all the MgO to Mg(OH) It should be noted that the slaking times were in excess of those actually required and taught by the present invention to insure complete slaking in all cases in order that correct surface area determinations of the Mg(OH) could be taken. Accordingly, the slaking times were extended to those times indicated.

Part (b) of each subportion of the precipitator slurry was also heated to the above temperatures for the above times, but without first filtering, for the purpose of treating the MgO to Mg(OH) in the presence of the CaCl mother liquor in accordance with the present invention. After such treatment the solids were recovered by filtration, similarly washed as in part (a), and then reslurried in water to a 50 percent solids level.

Each sample part (a) and (b) of subportions 1 through 5 after the above treatment were filtered, washed With water, and the filter cake dried at 130 C. overnight. A representative prepared (sample) portion of each cake was tested by a nitrogen adsorption method for surface area in square meters per gram and for apparent density in grams per cubic centimeter by the method described below:

Sample preparation. The dry sample was screened through a 28 mesh sieve. About 15 g. of each sample was pelleted at 15,000 p.s.i.g. using a commercially available laboratory press with a 1.125 inch diameter die.

Firing.--The samples were fired in a gas fired zirconia furnace along with standards in the following manner: 25 to 500 C. in 1 hour; 500 to 800 C. in 2 hours; 800 to 1000 C. in 1/2 hour: 1000 to 1,700 C. in 2 hours, hold at 1,700 C. for 1 hour, cool overnight.

Pellet evaluation.The pellets were crushed through a number 10 mesh screen and caught on a number 16 mesh screen. Apparent density was determined by a mercury porosirneter analytical method, modified to measure the volume of pores of less than 17.5 microns in diameter.

The results of these tests are categorized and presented in Table I below:

TABLE I.INITIAL PREGIPITATOR SLURRY 2% days at 100 C. 3% days at 80 C. 6% days at 65 0. 7 2 days at 45 C. i 15% days at 25 C.

Subportion parts a b a b a b a b a b Chloridcs on 50% Mg(OI-I)i solids basis 0. l4 0. l3 0. 14 0. 12 0. l4 0. 11 O. 14 0. 12 0. l4 0. 11 Periclase apparent density of calcined Mg(OH)z gnL/cc. (MgO dried cake) 3.17 3. 22 3.16 3. 22 3.15 3.21 3.17 3. 23 3.19 3. 22 Surface area in sq. meters per gram Mg(OH-)2 (double washed and dried) 7. 9 13.1 7. 2 13. 0 8.2 13. 5 9. 0 15.1 10. 2 14. 6

63 C. for about 5.6 hours, and at a soluble alkalinity It can be seen from the data in Table I for each of level of 5-6 mls. Soluble alkalinity means the number subportions 1 through 5 when comparing the data for of mls. of 0.1 N HCl necessary to neutralize 100 mls. (a) and (b) for each said subportion that the apparent of precipitator slurry filtrate to a phenolphthalein end density of the CaCl treated MgO in (b) as opposed to point. Magnesium hydroxide precipitate containing MgO 65 that of the water treated MgO in (a) is substantially was thereby formed dispersed in an aqueous mother greater in all cases. Moreover, the surface area of the liquor containing about 20 percent CaCl and from CaCl slaked MgO (Mg(OH) is significantly greater in which the aforesaid initial sample was taken for purposes every case. of the following tests: Accordingly, treating the precipitator solids in accord- A first portion (A) of the initial slurry sample was ance with the present invention produces a magnesium further divided in 5 subportions 1 through 5, each subhydroxide having a high surface area, and a resulting portion being further divided into two parts (a) and (b). magnesium oxide product characterized by a higher than Part (a) represents the control or prior art process (not ordinary apparent density when compared to the densiin accordance with the invention), whereas, part (b) is ties obtained from the prior art processes and control of illustrative of the invention. this example.

5 6 EXAMPLE II required and taught by the present invention to insure To Show that the novel Staking method of the present complete slaking in all cases in order that correct surface invention makes possible a dewaterable and repulpable area detemhhahohs the )2 Could be takenmagnesium hydroxide slurry having a significantly greater The efieet 0f Caclz coheentfe-tlon and treatment solids level, the filtering data for the subportions and parts Pefatufe 0n the Surface area in Square meters P gfafn of subportions in Example I is presented in Table II be- (M. /gm.) of the resulting Mg(OH) solids is shown in low. The 50 percent magnesium hydroxide slurries in parts Table III below:

TABLE III.EFFECI OF 0500 CONCENTRATION AND TREATMENT TEMPERATURE ON THE SURFACE AREA (ml/gm.) OF THE RESULTING Mg(OH) SOLIDS 0501i Gaol, 05012 C501 C2101; c501 c501 CaCh At 100 0. for 1 day 10. 5 11.3 12. 5 12.8 13.2 13.4 14.5 15.2

At 80 0. for 2 days 11.2 12.0 12.4 13.1 13. 7 13. 9 15. 4 15. 6

at 65 C. for 5 days.- 12.9 14.1 14.0 14.6 14.8 15. 7 16. 5 16. 5

At 45 C. for 7 days 12.8 14.2 13. 7 15.4 14. s 16. 7 16. 5 15. 0

At 0. for 12 days 14.0 14.0 14.0 14.7 14.5 15. 0 14.8 14. 7

(a) and (b) above were dewatered by a series of as many The extended time required to prepare the forty repetitive filtration and repulping steps as possible. Table samples permitted a larger than normal quantity of the II below indicates the number of filtrations possible for 20 MgO to slake in the presence of the concentrated CaCl each part (a) and (b), and the Mg(OH) solids content mother liquor. This condition reduced the spread in suriI1 Pefeeht y Weight derived from efleh- 011 the average face areas somewhat; however, by simple inspection of the the Caclz slaked material y be dewatered to a 4 D table it is apparent that the higher surface areas are obcent higher Mg(OH) solids content than the water slaked r tained at 25 45 5 and C h r f rr d treatmaterial. Moreover, Table II provides data which show 20 ment temperature is 45 to .0 C since at lower that the CaCl treated material is pumpable at a higher peratures the rate of Mgo S1 aki Hg is lower. This data sgig gggz fi i fiz 3 gi i s i si z i zg shows it to be apparent that concentrations above about pumpable was estimated by diluting each of the final filter E 6 gf prorFote hlgher Surfaces Wlth 10 30% cakes to various concentrations with water and establish- 30 a 2 emg 6 Pre erre range' ing the shear stress in pounds per square feet obtained at a shear rate of 100 seewith a Haake Rotovisco Viscom- EXAMPLE IV eter. A plot of the shear stress values versus percent I t I M (01- lid permits estimating the Solids content at To ll-ustrate the mlnrmum t1me requirement under the which the shearing stress is equal to 1.5 lbs./ft. Past eX- Invention to treat 2 p p 8011(18 11 perience has shown that at 1.5 lbs/ft. and lower values elllm ehlorlde, as Compared o t e minimum time rethe slurries are readily pumpable. quired in water, and to show the differences in surface TABLE II 2% days 3% days 6% days 7% days 15% days at 100 C. at 80 C. at 65 C. at C. at 25 C.

Subportion parts a b b a b a b a b Filtration data of adjusted slurry:

Percent Mg(OH)4 solids in first cake 60.8 63.3 61 6 63.3 61.2 63 4 60.3 62.6 59,3 62,0 Percent Mg(OH)2 solids in second cake." 67.9 68.8 68 9 70.4 68.4 71 0 68.0 70.3 67.3 68.8 Percent Mg(OH) solids in third cake 74.6 75.9 (I) (1) r) Rheology of Repulped Material, percent Mg(OI-I) solids level at which slurry becomes pumpable. 59.5 64.5 59.5 66.6 58.0 65 4 59 2 63.5 60 6 64 3 Point where shear stress falls to 1.5 lbs/it. at a shear rate of 100 sccr 1 No longer filterable.

It can readily be seen from Table II that in each case area of the solids, and in addition to illustrate that such the CaCl treated material produced a more concentrated diff r i l t bl to h treatment of h M a p p s1llflly Product than did the Water treated 'f fresh sample of precipitator slurry was split into equal terial. As calculated from t i d e p p Slumes portions. The first portion was filtered and the resulting y the method of the Present e h averaged 5% cake washed essentially free of the 20% CaCl mother percent highfg Solids t fl t l 1S h h t fi liquor with water. The washed filter cake solids were re- 01 esswaerin esur i gria t i z no h ii gl sh igp ng cost advantage is realized. Slumed or re SI-1Spended m a Small quannty of Water and concurrently with the second portion of the precipitator EXAMPLE III slurry (but separately) placed in a 100 C. water bath.

To illustrate the eifect of varying CaCl concentrations 1 samples were Withdrawn after and 50 in the treating liquid of the present invention, a number of 'mlhutes; 81/2, and 22 hours, a d a ter 2, 3, a d artificial mother liquors were prepared in which to treat 6 days, respectively. These individual samples were filthe MgO containing precipitator solids. tered, the cake washed with water, rinsed with alcohol to Th initial precipitator slurry sample Obta ne prevent further hydration of residual MgO, and the final yefltionany as in Example I filtered and the Solids cake dried at a temperature of 120 C. Each of the dried washed essentially free of CaCI The wet filter cakes were filter cakes was subjected to Surface area and percent theft transferred lhto Venous stl'ehgths CaclZ Solutlons ignition loss measurements. The latter analysis consists to yield afinal soluble concentratlon of 0, 0.5, 1, 2, 5, 10, of calcium" the dried Solids to 10004: C in a muflie 20, and 30 percent CaCl Each of the resulting slurries furnace fi Wei ht loss fl ct O l t f M 0 were then subdivided into five equal portions for treating h d h g re e F encss 0 g at temperatures of 100, 5 6 50, and 250 C for ra ion m t e prec1p1tator s01 s. Typlcally, the value periods of 1, 2, 5, 7, and 12 days respectively Again the W111 lncrease until all of the MgO has slaked (hydrated) slaking times were greatly in excess of those actually and then level off.

1511 Portion (In 2nd Portion (In Percent Surface Percent Surface Ignition Area, Ignition Area,

Contact Time Loss mfi/gm. Loss mfl/g-m.

8% hours- 29, 7 l0. 1 30. 5 14. 4 22 hours 30. 3 9. 7 30. 7 14. 7 2 days 30. 6 9. 1 30. 8 14. 6 3 days 30. 6 8.9 30. 6 l5. 0 6 days 30. 7 8. 9 30. 9 l5. 4

The drawing portrays this data graphically, in FIGS. 1 and 2.

As is clear from the data in Example IV, hydration of the MgO precipitator solids is essentially complete in 4 to 8 hours of contact time at 100 C., and the final surface area of said solids is significantly greater in the CaCl treating solution than in water and is substantially constant after about 4-8 hours of contact time.

The present invention may be modified or changed without departing from the spirit or scope thereof, and it is understood that the invention is only limited as defined in the appended claims.

We claim:

1. In a method of preparing magnesium hydroxide from dolomite and a brine wherein (a) slaked dolime comprising calcium hydroxide and magnesium oxide is reacted with an aqueous magnesium halide-calcium chloride-containing brine, thereby obtaining an initial magnesium hydroxide precipitate, and an aqueous calcium chloride-containing mother liquor which also contains solid magnesium oxide from said dolime, (b) the magnesium hydroxide and magnesium oxide solids are separated from said mother liquor, (c) subsequently the magnesium oxide solids are water slaked to form additional magnesium hydroxide, and (d) all the magnesium hydroxide is separated from the slaking solution; the improvement which comprises substituting for steps (b), (c) and (d), after step (a) the following steps:

(1) treating the magnesium hydroxide and magnesium oxide solids by extended contact with said mother liquor at a temperature of from about 25 C. to about 100 C. for a time of from about 4 to about 120 hours; such time and temperature being sufficient to convert substantially all the magnesium oxide to magnesium hydroxide; said treatment being essentially completed prior to any separation of said solids from said mother liquor; and (2) separating essentially all the magnesium hydroxide from the mother liquor. 2. The improvement of claim 1 wherein the treatment temperature is from about to about C.

3. The improvement of claim 1 wherein the mother liquor has a calcium chloride content of from about 10 to about 30 weight percent.

References Cited UNITED STATES PATENTS 3,007,776 11/1961 Periard 23-201 3,080,215 3/1963 Waldron et al. 23-201 3,294,485 12/1966 Mayer 23-201 X 3,301,633 1/1967 Stowe et al. 232O'1 X 3,366,451 1/1968 Waldron et a1. 23201 OSCAR R. VERTIZ, Primary Examiner.

G. T. OZAKI, Assistant Examiner. 

